The purpose of this work is to assess mental health and mental health service utilization over time among a representative sample of Reserves and National Guard within a multivariate causal framework that takes into account life course experiences and circumstances together with combat history, other Reserve and National Guard experience (e.g. humanitarian activity) and civilian traumatic event experiences as determinants of mental health in this group. In particular we focus here not only on documenting the prevalence and correlates of mental illness and health service utilization among these forces, but also on documenting the trajectories of mental health and well- being over time among these forces. Although there is a growing literature about the mental health and mental health needs of active duty military personnel, this would be the first study, as best we know, that has focused explicitly on the experience of the Reserve and National Guard. This is particularly important given the growing reliance on Reserve and National Guard forces during the war against terrorism in Iraq and Afghanistan. Current deployment to one of these two countries as parts of Operation Enduring Freedom (OEF) or Operation Iraqi Freedom (OIF) have added to the duties of members of the Reserve and National Guard forces in the US, including frequently deployment as part of the response to natural disasters. For example, during the response to Hurricane Katrina, the military deployed 72,000 forces including 50,000 National Guard as well as Navy ships and helicopters. In this study we propose to document psychopathology, health risk behavior, and mental health service utilization among National Guard and to identify personal factors (e.g., sociodemographic factors, social supports), Reserve and National Guard involvement (e.g., length of time in service, unit cohesion, extent of training, rank, separation from family/loved ones), and life circumstances (e.g., integration of regular work circumstances) that are associated with both stage- sequential psychological morbidity and trajectories of psychological morbidity over time among Reserve and National Guards when accounting for combat exposures (e.g., international deployment, active duty combat) and other traumatic event experiences. This study has implications for early intervention after exposure to traumatic events (including combat experiences and domestic deployment), training of National Guards and Reserves, and education of commanding officers and military leadership. In a typical year, US military forces, including many members of the Reserve and National Guard, are deployed in over 100 countries around the world. The reliance on Reserve and National Guard forces has become ever greater during the war against terrorism in Iraq and Afghanistan. The purpose of this work is to assess mental health and mental health service utilization over time among a representative sample of Reserves and National Guard that takes into account life course experiences and circumstances together with combat history, other Reserve and National Guard experience (e.g. humanitarian activity), and civilian traumatic event experiences as determinants of mental health in this group. This study can help guide the development of early intervention after exposure to traumatic events (including combat experiences and domestic deployment), training of military personnel, and education of commanding officers and military leadership about ways to improve resilience among Reserve and National Guard forces.